Investigating Changing-Look Active Galactic Nuclei with Long-term Optical and X-Ray Observations

TL;DR

This study analyzes 20 changing-look AGNs over 40 years, revealing that their spectral transitions are linked to changes in accretion rates rather than obscuration, and occur on timescales shorter than expected viscous timescales.

Contribution

It provides the first long-term optical and X-ray observational analysis of CLAGNs, linking spectral state changes to accretion flow instabilities rather than obscuration.

Findings

CL transitions are accompanied by changes in Eddington ratio.

Transitions are not associated with obscuration changes.

Most CL events occur within 3-4 years.

Abstract

Changing-look active galactic nuclei (CLAGNs) show the appearance and disappearance of broad emission lines in their UV/optical spectra on timescales of months to decades. Here, we investigate how CL transitions depend on several AGN parameters such as accretion rate, obscuration properties and black hole mass. We study a sample of 20 nearby optically-identified CLAGNs from the BAT AGN Spectroscopic Survey (BASS), using quasi-simultaneous optical and X-ray observations taken in the last $\sim 40$ years. We find that for all CLAGNs, the transition is accompanied by a change in Eddington ratio. The CL transitions are not associated with changes in the obscuration properties of the AGN. CLAGNs are found to have a median Eddington ratio lower than the AGNs in the BASS sample in which CL transitions were not detected. The median of the transition Eddington ratio (Eddington ratio at which AGN changes its state) is found to be $\sim 0.01$ for type 1 $\leftrightarrow$ 1.8/1.9/2 transition, which is consistent with the hard $\leftrightarrow$ soft state transition in black hole X-ray binaries. Most CL events are constrained to occur within 3-4 years, which is considerably shorter than the expected viscous timescale in AGN accretion disk. The transitions of the optical CLAGNs studied here are likely associated to state changes in the accretion flow, possibly driven by disk-instability.